/3/ - 3DCG

I'm having a weird problem in Blender where sometimes pinning one vertex somewhere will make it act like the whole island in pinned when unwrapping (i.e. nothing happens to that island at all, but all other islands get unwrapped and ignore its existence)
I have no idea what's causing this. I tested the same blend in a new install, the monkey in the original install, and the monkey in a new install, and it happens with all of them. My only explanation is that I'm somehow retarded and forgot how pinning works? I've definitely pinned vertices in place before and unwrapped the island around them, though, so I know it's possible. Maybe I'm just missing some crucial step?